Electronic tone signal generators



Aug. 15, 1961 Filed Jan. 5l, 1958 B. R. LAWRENCE ELECTRONIC TONE SIGNAL GENERATORS 2 Sheets-Sheet 1 Flg l sTG.| 5TG.2 STG.5 sTG.4 ST55 STG.5 STGJ STG.8 SW1 f L Y A Y A T A y L .T A w i L v g` B+ I l R5 R5 R R5 Rs R5 R5 {RS T l R9 R9 R9 R9 R9 R9 R9 g 1 T Nw 'Vx/w 'Ww ww W Nw I l i `1 l 1 RS RG RG RG Re Re Reg: Rs: i I l [b2 2 T2 p2 2 p2 nja [F2 2 aga T2 l L 1 1 T 1 l l c2= C2" C22 c2== o2 c2 C2" il 02"? 1 1 1 1x: 1: f 'l i? R7 R? R7 R? R7 R7 R? i R? ,Re

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ELECTRONIC TONE SIGNAL GENERATORS Filed Jan. 51, 195e 2 sheets-sheet 2 INVENTOR United States Patent O (501 Townville St., Seneca, S.C.)

Filed Jan. 31, 1958, Ser. No. 712,474 5 Claims. (Cl. 331-51) My invention relates to improvements in electronic tone signal generators used in electronic organs and particularly to that type which generates an electric signal corresponding in frequency to each note in the diatonic tempered musical scale.

It is customary in all-electric organs which employ the Formant principle of tone coloring to generate a series of sawtooth waves which are passed through fixed, selectable electric filters to modify the waveshapes and reproduce various tone colors. The Formant may be defined as a frequency range in which the harmonic components of a complex wave are prominent relative to the harmonies in other frequency ranges. This system requires that a fundamental tone containing all of its harmonics in the audio range be supplied to the filters. The frequency response of the filters acts to emphasize certain bands in the spectrum in a manner similar to which `various musical instruments do due to their physical composition and configuration. The sawtooth wave is generated for this purpose because it contains all succeeding harmonics with amplitudes inversely proportional to their orders.

It is customary to generate the electric tone signals approximating a theoretical sawtooth wave, by use of vacuum tube oscillators, gas tube oscillators, thyratron oscillators, or the like. The oscillators are usually divided into 12 groups, 11 of which contain 7 oscillators and one of which contains 8, in order to complete the seven octave range required for 4', 8', and 16' organ stops. Each of these groups employs a master oscillator tuned to the frequency of one of the 12 notes in the high register octave, between 2349 and 4186 cycles per second, and the corresponding note in each succeedingly lower octave is obtained by frequency division and/or electrical synchronization with the master oscillator.

The customary practice of electronic tone signal generation possesses disadvantages. The use of vacuum tubes is grossly inefficient in electric power consumption, generates a large amount of heat, and requires a large amount of space. A further disadvantage is the lack of precise frequency control. In the various systems in use, all are affected by line voltage variations and frequency drifts due to changes in the electric characteristics of components, i.e., condensers, inductors, and vacuum tubes, caused by the effects of temperature, humidity, and aging. While this frequency drift is slight, very high quality and expensive components are required to minimize it and the frequency is never precise. Further, the purity of the sawtooth wave signal and its degree of approximation to a theoretical sawtooth signal is limited. The response times of the components and circuity are relatively slow, resulting in smoothed sawtooth signals which do not contain all the harmonics in proper proportion.

The object of my invention is to provide an electronic tone generation system of high power efficiency, accurate frequency control, and with improved purity in the harmonic content of the generated tone signal.

More specifically, a principal object of my invention is to provide frequency control with the preciseness of a fine steel reed.

A further object of my invention is to improve the power efficiency of electronic tone generation by a minimum factor of l0, and to greatly reduce size and weight.

Still another objet of. my invention is to improve the ice purity of the harmonic content of the sawtooth electric signal to more closely approach the theoretical sawtooth.

In the accompanying drawings, forming a part of this application, and in which like letters and numerals are employed to designate like functional parts thoughout the same,

FIGURE 1 is a complete schematic drawing of the Tone Generator and Frequency Control Sections showing the manner of connection of their components represented by standard symbols,

FIGURE 2 is a three-quarter view of the preferred configuration of the adjustable reed-tuned frequency control transformer.

In FIGURE 1, TZ designates a double-based diode, also known as a unijunction transistor such as the General Electric 2N492 or the like, consisting of a silicon bar with ohmic contacts base 1 and base 2 at each end, and an aluminum wire connected to one side of the bar -forming a p-n junction which is called the emitter.

In FIGURE 1, tone generator section, the circuit con* sists of seven (or eight) stages identical in circuitry and function. There are twelve tone generator sections in the complete organ tone generator assembly, eleven with seven stages, plus one of eight stages (C note) to complete the notes of the organ range. When SW1 is closed to connect an external D.C. power supply to the tone generator section shown, current flows into the two power connections in stage 1. The supply voltage is divided between R5, a small resistance in the order of 1500 ohms, and transistor T2, the preponderant portion of the supply voltage being distributed uniformly between base 2 (top) and base 1 (ground) of the transistor, a resistance in the order of 6000 ohms. The voltage in the bar at the emitter junction of T2 is in the order of 0.6 of the base 2 to base 1 voltage. At the same instant the supply SW1 is closed current flows through R6 charging C2 at a rate determined by the R6-C2 time constant. No appreciable current ows into the transistor bar through the emitter junction as long as the emitter voltage is below the voltage in the bar at the junction because the emitter-base 1 resistance in this state is very high. When condenser C2 is charged to a voltage slightly more than the bar voltage at the junction current will flow into the junction and down to base 1. This current increases the minority carriers in the region from the emitter to base 1, decreasing the resistance of this portion of the bar which encourages more current which lowers further the resistance etc., etc., resulting in a regenerative action. The negative resistance in effect switches the device from off to on in a few microseconds. The regenerative action continues until the bar below the emitter is saturated with minority carriers. The condenser is discharged through this low resistance and when current flow stops the transistor returns to its original off state. The cycle now repeats itself at a rate determined by the capacitor C2 charging and discharging time constants. The discharge time, a few microseconds, is extremely small relative to the charging time which is in hundreds of microseconds at the highest frequencies involved. Therefore the sawtooth voltage wave form at the emitter (junction of R6 and C2) closely approaches a theoretical sawtooth wave form. This signal voltage is connected to ground through Voltage divider R7 and R8 and the output signal is taken from the junction between R7 and R8. This voltage division reduces the strength of the signal from the order of 12 volts peak-topeak, with a 30 volt supply, to the order of 2 volts needed for the following circuits in an electronic organ of this type, reduces the source impedance of the signal to a low value, and reduces loading on the R15-C2 circuit to a negligible level. The output impedance across R7 may be shorted to ground with no effect on the frequency of the stage, as will be explained later. Stages 2 through 8 function in a similar manner, the value of R6 and C2 being fixed to provide the approximate desired frequency of each stage respectively.

At the time C2, stage 1, of the Tone Generator Section, FIGURE 1, discharges through the emitter of T2 the minority carriers introduced into the bar reduce the resistance in the bar between the emitter and base 1 and the total resistance between base 2 and base 1 is also reduced sharply. A pulse of current flows from base 2 to base 1 during these few microseconds and the same current pulse flows through R5 causing a sharp negative voltage pulse at the connection R5-T2. This pulse occurs each time C2 discharges and therefore at the identical frequency of the generated sawtooth tone signal. These pulses are transmitted at an attenuated magnitude through R9 from stage 1 to stage 2. Whenthe power is connected through SW1 the condensers C2 in stage 1 and stage 2 begin charging, but the R6-C2 time constant of stage 2 is slightly greater than double that of stage 1. The transistor T2 in stage 1 switches on when the condenser C2 in stage 2 is roughly half the voltage required to switch this stage. The pulse generated at the base 2, stage 1, is transmitted to base 2, stage 2, and does decrease the base-to-base voltage of T2, stage 2, also lowering the emitter junction voltage but not to a value below the emitter voltage and, therefore stage 2 will not be fired by this first pulse. However, when stage 1 fires the second time, C2, stage 2, has now charged up to a point where the emitter voltage is above the junction point voltage at the instant it is lowered by the pulse reaching base 2, stage 2, from stage l, and stage 2 switches on for the first time at the same instant stage l switches its second time. The cycle repeats itself from this point with the result that stage 2 is switching at exactly one-half the frequency of stage 1, and the sawtooth tone signal is locked in phase exactly one octave below that of stage 1. In exactly the same manner stage 3 is locked to a frequency one octave below stage 2, and so on down the stages consecutively so that one generator produces seven (or eight in the case of C) octavely related notes positively locked together in phase. The sync pulse coupling resistors R9 are adjusted so that pulses transmitted are of a magnitude just shy of that required to fire the next lower stage at its half-cycle point so that there is always ample sync pulse magnitude at the full-cycle point even under conditions of full loading. Actually any of the output terminals can be grounded without loss of positive synchronization. The pulses from stage l also reach stage 3 but are at this stage attenuated to a degree that they do not affect its synchronization whatsoever. Likewise no stage is affected by any sync pulses except by the pulses from the stage immediately above it in frequency.

This means of synchronization is one of the novel features of my invention. It is an important improvement in that the sync signals are so well isolated from the electrode from which the output tone signals are taken, the T2 emitter in this case. As long as the emitter is back-biased the voltage changes in the transistor bar from base 2 to base 1 cause current changes in the emitter in terms of only a few microamperes. This, from a practical standpoint, has no detectable effect on the output sawtooth signal at the emitter. The simplicity and use of inexpensive parts is also noteworthy.

In the drawings, FIGURE 2, wherein for the purpose of illustration, is shown a preferred embodiment of the frequency control transformer which is an essential part of my invention. The numeral 10 designates the frame portion to which all other parts are mounted, it being made. by punching from a flat plate, and made of a strong non-magnetic material such as brass. The frame includes mounting lugs 15 and 24V which are formed as a part of the frame plate when flat and bent upward at an `angle of 90; mounting lugs26, which are punched from the frame plate while at and bent upward at an angle of and mounting lug 28 which is formed as a part ofv the frame plate while fla-t and bent upward at an angle of 90. Fixed by permanent adhesive bonding tot the mounting lugs 15 and 24 and centered vertically are Alnico V type permanent magnet bars 14 and 25, whose' north poiles face in the same direction. Also-bonded to lugs 15 and 24 and to both ends of the permanent magnets are the four soft iron pole pieces 13, 16, 20, and 21. The primary coil windings 18 and the secondary coil windings 22 are wound of copper wire on plastic spools, slipped on the pole pieces and cemented in place prior to mounting the pole pieces. The length of the poile pieces are fixed to leave small air gaps 19 through which the tuning reeds 27 projects. A non-magnetic wing nut bolt 12 extends through holes in the mounting vlugs 26, the nonmagnetic tuning reed hold blocks 11, `and an elongated hole in the tuning reed 27 so as to hold the reed firmly and align it in the center of the pole piece air gaps. The reed forms a common ferrous metal core to complete the ferrous metal loops in both the primary and secondary core paths but leaving small air gaps at the ends of the pole pieces. Extending through a threaded hole in mounting lug 28 is the non-magnetic wing nut reed tuning screw 29, having lock nut 30 To the end of the tuning screw is fixed a small circular disc 31 which fits into a slot 32 cut in the rearward end of the tuning reed 27. If the reed holding bolt 12 is loosened slightly, adjustment of the reed length projecting through its mounting blocks 11 is done by turning wing nut 29, whichv changes the natural resonant vibrating frequency of the reed. This feature provides for fine tuning. Although the reeds are ground from fine steel to` the resonant frequency desired, fine tuning is desirable for exact tuning and to allow the organ to be tuned to match other musical instruments if desired. This tunable feature is a novel feature of my invention which to my knowledge has never been used in conjunction with a vibrating reed device with high frequency stability. The connections of the primary coil terminals 17 and the secondary coil terminals 23 and the electro-mechanical operation of the transformer are depicted in the following paragraph.

In FIGURE 1 of the drawings is depicted the schematic detail of the frequency control section of my invention. T1 is lan audio frequency NPN transistor of a type similar to the Texas instrument 2N14-8A or the rlike. Resistances R1 and R3 comprise a voltage divider for the base bias of the transistor which is connected between them. R4 is the emitter bias resistor, by-passed for A.C. by condenser C1. The primary of the frequency control transformer consists of permanent magnet 14, north pole piece 16, south pole piece 13, and the pole piece coil windings 18, which are wound and connected so that the direct current flowing through them increases the magnetic flux already induced by the permanent magnets. In other words, the flux density `at the poles is increased rather than decreased by the steady state mean direct current in the coil windings. The secondary consists lof permanent magnet 25, north pole piece 20, south pole piece 2.1, and the pole piece coil windings 22 which likewise are so wound and connected that the direct current through them enhances the magnetic flux established by the permanent magnet. Assuming only direct currents in the coil windings, the magnetic flux emanating from the primary north pole 16, down through the reed 27, and into the south pole 13, exerts a force on the reed 27 which would tend to defiect it toward the primary north pole 16.

However, this force is offset by the flux emanating fromA the secondary north pole 20, down through the reed, and into the south pole 21, and under assumed conditions the reed would not be appreciably defiected. The .total resistance of the current path through R5 of stage 1, the prin-tary coil windings 18, and the transistor collector circuit is less than the resistance through R1, R2, and the secondary windings. Therefore, when the power SW1 is closed therateof buildup of magnetic fiux inthe primary is greater than in the secondary which results in a detlection of the reed 27 toward the primary north pole 16. When the reed is deflected away from the secondary poles 20 and 21 the air gaps in the secondary loop are increased in length, the magnetic reluctance of the secondary magnetic loop increases and the flux density tends to decrease. This change of flux induces a current in the secondary windings which flows through R2 into the base of transistor T, and the current in the collector is increased by the current gain factor of the transistor. This 'current fiows through the primary coil windings, increases the primary flux density, exerts a greater defiecting force on the reed and lche secondary flux is still further reduced resulting in a regenerative condition. This action continues until the current in the collector of transistor T1 approaches saturation and at the same time the restoring spring force of the reed begins to reverse its movement toward the secondary north pole 20. Ail currents and magnetic flux changes are now reversed and la regenerative condition exists in the opposite direction. It continues until the transistor approaches a cut-off condition at which point the cycle repeats itself and oscill-ation is begun. The sustaining level of positive feedback is established by a voltage divider consisting of R2-R'3 in the input circuit of the transistor. The mechanical resonant frequency of the reed 27 determines the frequency of oscillation and the A.C. component of current in the coil windings is roughly an alternating pulse wave. The current fiowing through the primary windings also fiows through base 2 resistor R5, stage l. The negative half cycle of this current switches the transistor T2, stage 1, in the same manner explained in preceding paragraphs with respect to synchronizing the u nijunction with negative pulses in its base 2 circuit.

The initiation of the reed vibration and transformer oscillation is assured by the arrangement of the circuit. The primary coil winding is connected in series with the base 2 resistor R5, stage 1. Oscillation of this tone generator stage as well. as the other stages, is certain due to the thermionic electron activity of its components. The negative pulses are occurring at R5 of this stage at a frequency very near the control reed frequency (slightly lower), inducing pulses of current through the primary coils at this frequency. Als a result small defiections of the reed are always induced at very near its resonant frequency and regenerative oscillation is assured.

The permanent magnets establish a bias flux density in the pole pieces which is of a magnitude sufficient to override the electromagnetic flux induced by the currents in the coil windings. Therefore the electromagnetism can never reverse the direction of flux, but only increase or decrease it. If reversal of fiux were allowed to occur in the reed it would receive two force impulses per electrical cycle instead of one, as the force impulses are independent of the direction of magnetic ux, and, as a result, the device would not operate in the desired manner. This iiux bias also reduces the undesirable effects of the hysteresis characteristics of the soft iron pole pieces.

Of course it is apparent to those proficient in the art that the lfrequency control oscillator provides an excellent and economical source of variable and stable alternating electric pulses for control of various external electric oscillator circuits. In addition the frequency control oscillator coupled to the first stage tone generator circuit provides a source of sharp negative pulses which may be obtained at the base 2 connection of the unijunction transistor, as well as sharp positive pulses obtained by the addition of a small resistor between the base 1 connection and ground. This combination provides a variety of sync pulse shapes at very low source impedance, is simple to adjust, and is stable `for long periods of time.

The important improvements I have herein described are summarized:

First, a new method of tuning and maintaining the frequency of a multiplicity of transistor oscillators and a new method of controlling the frequency of various electric oscilla-tor circuits in the audio range.

Second, an improved method of generating a series of sawtooth-wave voltages locked together in phase at octave intervals `for electronic organs, a method affording a great saving in power consumption, a great reduction in undesirable heat generation, space, and weight.

Third, the tolerances of the components are not critical, only standard production tolerances being required, and no expensive inductances or transformers are used. The simple windings on the frequency control transformer can be produced cheaply.

What is claimed is:

l. In an electronic tone generator, a plurality of oscillation circuits operative to produce electr-ical signals of predetermined frequencies, each of said oscillators comprising a semi-conductor element having a semi-conductor body provided with an ohmic contact at each end thereof and a junction electrode intermedi-ate said ohmic contacts, one of said ohmic contacts being connected to ground potential, the other of said ohmic contacts being serially connected to a first resistor and to a source of positive potential, frequency determining elements comprising a second resistor connected between said positive potential and the electrode, and a capacitor connected between the electrode and ground potential, said frequency determining elements operating to effect a periodic disruptive discharge of the semi-conductor element, and means -for synchronizing the relative disruptive discharges of the semi-conductor element of said plurality of oscillation circuits, said last named means including a resistor connected between succeeding ones of said oscillator circuits at the junction of said rst resistor and said other of said ohrnic contacts in each of said circuits, whereby a negative-going pulse generated in each oscillation circuit upon the disruptive discharge of its semi-conductor element is applied to the next succeeding oscillator circuit to synchronize the disruptive discharge relationship in said next succeeding oscillation circuit with the disruptive discharge in the oscillation circuit wherein the pulse was generated.

2. In an electronic tone generator, a plurality of oscillation circuits operative to produce electrical signals of predetermined frequencies, each of said oscillation circuits comprising a semi-conductor discharge element serially connected to a resistive impedance and to a source of positive potential, frequency determining elements connected to said discharge element for effecting a disruptive discharge of the discharge element at a predetermined rate, and means for synchronizing the disruptive ydischarges in said plurality of oscillation circuits with a basic frequency generator, said last named means comprising a mechanically resonant frequency device electrically connected to a first of said plurality of oscillation circuits at the junction of said discharge element and resistive impedance, said mechanically resonant frequency device being operative to produce a varying potential signal in accordance with the `frequency of its mechanical resonance, said varying potential signal being operative to induce a disruptive discharge of said discharge element in said rst of said plurality of oscillation circuits, and a resistive impedance connected between successive oscillation circuits at the junction of said discharge element and resistive impedance in each of said circuits, whereby a negative-going pulse generated in each oscillation circuit, including said first circuit, upon the disruptive discharge of its discharge element is applied to the next succeeding oscillation circuit to synchronize the relationship of disruptive discharges of said oscillation circuits.

3. An electronic signal generator comprising an oscillator circuit including a semi-conductor discharge element, a first resistor connected between said discharge element and a source of positive potential, :frequency determining elements connected to said discharge element for effecting a disruptive discharge of said discharge element and means connected to the junction of said discharge element and resistor for controlling the disruptive discharge of said `discharge element, said last named means comprising a mechanically resonant generator having a natural frequency of vibration equal to the basic frequency of the electronic signal generator, said rnechani" cally resonant generator including a pair of spaced opposed magnetic pole pieces, a tunable mechanical reed positioned between said pole pieces, a transistor amplifying device having a base, collector, and emitter electrode, a first winding positioned on a rst of said pole pieces and connected on one end to ground potenti-al and on its other end through a second resistor to the base electrode and said source of positive potential, a second winding positioned on the other of said pole pieces and conneeted on one end to said emitter electrode and on its other end to said source of positive potential through said trst resistor, and biasing means for said transistor device connected between the collector electrode and ground, whereby energization of said mechanical resonant generator produces a vibratory motion of said reed and a pulsating electrical signal having a yfrequency of said vibrating motion, said signal being operative to control the disruptive discharge of the discharge element.

4. In the apparatus as defined in claim 2, means for adjusting said tunable mechanical reed comprising a base support, a pair of opposedflanges positioned perpendicular to and attached on 4 said support, spaced mounting blocks positioned between said anges and adapted to receive said reed intermediate its ends, means for securing said reed between said blocks, an adjustment support attached perpendicular to said base and positioned normal to one end of said reed, elongated threaded means movably mounted in said support with attachment means adjacent one end thereof for engaging said reed, whereby movement of said threaded means is operable to move said reed relative to said blocks for tuning the same.

5. In an apparatus as dened in claim 2 having a series of oscillation circuits of the type described and each being operative to produce electrical signals having lfrequencies related by multiples to each other, means for synchrovnizing the disruptive discharges of the discharge element inl each of said series of oscillation circuits with said mechanical resonant generator, said last named means comprising a resistor connected, in turn, between successive oscillator circuits beginning with said oscillator circuit controlled -by said mechanical resonant generator, said connection being made at the junction of said discharge element and the first resistor in each of said circuits.

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